A laser is an optical source that emits photons in a coherent beam. Laser light is typically a single wavelength or color, and emitted in a narrow beam. Laser action is explained by the theories of quantum mechanics and thermodynamics. Many materials have been found to have the required characteristics to form the laser gain medium needed to power a laser, and these have led to the invention of many types of lasers with different characteristics suitable for different applications.
The color or frequency of the emitted light may depend on the characteristics of the gain medium. However, not all desired colors have gain mediums found to reliably reach the color desired. Another method of generating a particular color is called frequency doubling. In frequency doubling, a fundamental laser frequency is introduced into a nonlinear medium and a portion of the fundamental frequency is doubled. Frequency doubling in nonlinear material, also called second harmonic generation, is a nonlinear optical process, in which photons interacting with a nonlinear material are effectively combined to form new photons with twice the energy and, therefore, twice the frequency and half the wavelength of the initial photons.
A polarized material is a material in which light exhibits different properties in different directions within the polarized material. The polarization of a material may make a significant contribution to the nonlinear optical process, since polarization results in charge oscillations within the medium. When laser light propagates in a medium, the associated electromagnetic field generates patterns of electric and magnetic polarization of the medium, which propagate together with the generating optical wave. The interaction of the electric polarization wave with the electromagnetic wave reduces the phase velocity of the combined phenomenon below the vacuum velocity of light. This is termed the index of refraction. The index of refraction is the factor by which the phase velocity is reduced.
One method of producing SHG light provides for a fundamental light, from a seed laser, that may be focused into a nonlinear crystal. Upon traversing the nonlinear crystal in a single pass, the remaining fundamental light is filtered out of the second harmonic output. This method may be an inefficient use of the fundamental light and likely an inefficient utilization of costly nonlinear material.
Another method of generating second harmonic light employs a resonant nonlinear material structure, in other words an intra-cavity SHG. Using this method, the nonlinear material structure is within the lasing cavity of the seed laser. Fundamental light that survives the nonlinear structure without conversion provides feedback to the seed laser. The intra-cavity SHG laser may be difficult to align and control, which may cause instability in the output of the seed laser.